Methods for producing lactic acid that are currently performed industrially are based on lactic fermentation of saccharides (see Patent Literature 1). However, such methods are problematic in that cellulose should be subjected to a saccharification step using acid, an enzyme, or the like so that cellulose can be used as a raw material for lactic fermentation. Also, in general, lactic acid production methods based on fermentation are problematic in that the reaction rates are low, huge fermentation tanks are required, the concentrations of the thus generated lactic acid are low, and the resulting energy consumption for purification is high. Furthermore, the solution pH decreases as lactic fermentation proceeds, resulting in decreased lactic acid bacteria fermentation efficiency. Accordingly, lactic fermentation is performed while neutralizing the solution with a base. Therefore, the product generated by the lactic fermentation methods is lactate, lactate is treated with acid to liberate lactic acid from lactate, and then the treatment of a neutralization salt resulting therefrom also poses significant problems in terms of processes.
As a lactic acid production method not based on any biological technique, a chemical method is known that involves treating carbohydrates hydrothermally in the presence of an alkali. For example, when a saccharide (see Non-Patent Literature 1 and Non-Patent Literature 2), cellulose (see Patent Literature 2 and Non Patent Literature 3), or organic waste (see Non-Patent Literature 4) is treated by the method, some carbohydrates that have been degraded under high-temperature and high-pressure reaction conditions are isomerized to generate lactic acid. However, the method is problematic in that lactic acid reacts with the alkali added as a catalyst to result in lactate, some inorganic acids must be added to the reaction solution to make the solution acidic for separation of lactic acid as acid, and thus alkali and inorganic acid are stoichiometrically consumed.
As a chemical method for producing lactic acid without using an alkali, a method has been reported that involves reacting starch, oligosaccharide, or monosaccharide with alcohol using a metal halide as a catalyst, so as to convert it into lactic acid ester (see Patent Literature 3). However, as a result of examination by the present inventors, a cellulose-based raw material could not be degraded by this method at less than 200° C., and generation of lactic acid or lactic acid ester could not be confirmed.
An example has also been reported wherein a cellulose-based raw material is directly converted to lactic acid by a chemical reaction without using an alkali. However, the method requires very high-temperature and high-pressure (temperature of 350° C. or higher and less than 400° C., and pressure of 20 MPa or more to 35 MPa) reaction conditions, and thus energy consumption is high and the yield of lactic acid is insufficient (see Patent Literature 4).
As other reports concerning lactic acid production from a cellulose-based raw material within a single step, an example of using a group III metal salt as a catalyst (see Patent Literature 5 and Patent Literature 6) and an example of using a rare-earth metal oxide as a catalyst have been reported (see Patent Literature 7). These methods exhibit high lactic acid yields only under conditions of a relatively low concentration of a raw material. A production method that exhibits a good lactic acid yield with a higher concentration of a raw material is desired from a practical standpoint.